connmark: Fix undefined shift if traffic selector is /0

Same as the previous commit for consistency, but not an issue here as
only transport mode SAs are handled.

Fixes: b8973b2661 ("connmark: Add CONNMARK rules to select correct output SA based on conntrack")

src/libcharon/plugins/connmark/connmark_listener.c

@@ -64,7 +64,7 @@ static bool ts2in(traffic_selector_t *ts,
 	{
 		memcpy(&addr->s_addr, net->get_address(net).ptr, 4);
 		net->destroy(net);
-		mask->s_addr = htonl(0xffffffffU << (32 - bits));
+		mask->s_addr = bits ? htonl(0xffffffffU << (32 - bits)) : 0;
 		return TRUE;
 	}
 	return FALSE;